In ducted fan unmanned aerial vehicles (UAVs), struts are used to provide structural support between the duct and the center body. The flow induced by the fan into the duct produces noise due to a number of flow and structure interaction mechanisms. One of the most dominant noise sources in ducted fan UAVs is the strut-induced flow disturbance that interacts with the fan's rotors. The two primary strut-induced flow disturbances are: (1) the wakes produced by air flowing over each strut and (2) the counter-rotating vortices generated at the strut-duct junction. These two sources of noise hinder the stealth characteristic of ducted fan UAVs, which could result in premature location of the vehicle due to an excessive noise signature. Thus, being able to reduce the noise generated by the ducted fan will allow UAVs to be utilized in more military and stealth applications.
A ducted fan UAV relies on a significant volume of airflow through the duct to generate the needed lift for the vehicle to fly. As such, significant structural support is needed in the duct to hold the fan centered in the duct and to support the engine. The design and placement of these support struts, thus, is critical to not only the structural integrity of the vehicle but also to the acoustic and noise performance of the ducted fan. The conventional approach to implement the struts is to place them inside the duct in a radial straight configuration. This configuration is particularly loud.
Further, for a UAV, such as a micro-air vehicle (MAV) or Class I vehicle, the center of gravity must be centered in front of or above the duct lip. This requires that significant weight be centered above the vehicle duct and that structural elements be in place to support that weight. In the MAV, those structural supports or struts are located in the full flow of the duct stream, generating significant noise.